I've got a Canon 70D that I bought Summer 2015. I've been shooting time-lapse of the construction of my friends' new house, and today I noticed that there is a weird overexposed shape in all of the images. I initially thought that maybe the sun was blazing through the viewfinder somehow, so I put black electrical tape over it; didn't help.

Then I thought maybe it was the lens, but I've tried all 3 of my lenses, and the problem remains. I even took some pictures without a lens at all, and the shape still shows up.

I think it must be an issue with the sensor, but the shape doesn't show up in videos, or on the live-view digital screen. If I take a picture from live-view though, then it shows up in the captured image.

@FotisK I don't know what you mean by "the prism not turning fully" -- the prism of a DSLR doesn't move in any way. And the asker has already said that they tried taping over the viewfinder to avoid any potential of light leaks from there.
– David RicherbyFeb 2 '17 at 8:27

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@FotisK No worries. Shall we delete these comments, since they just duplicate your answer and you've corrected your answer?
– David RicherbyFeb 2 '17 at 17:50

3 Answers
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It looks like you've got a problem with the shutter. Possibly one of the "blades" in the second curtain is missing and allowing light to strike the sensor between the time after the second curtain has completed its travel and the time the sensor is read out.

Here's what a 70D shutter curtain looks like. Does the shape of the 2nd of four individual blade that make up each of the curtains look familiar to you?

It is fairly easy to confirm. Just set your camera for manual exposure with a fairly long shutter time. Say 1 or 2 seconds. Enable mirror lockup. Remove the lens, point the camera so that you can see into the front of the mirrorbox, and press the shutter button all of the way down. This will raise the mirror while leaving the shutter closed. The first curtain should be entirely covering the sensor. If there's a gap anywhere that lets you see part of the sensor at this point then the first curtain is faulty. Press the shutter button again to expose the sensor. After the first shutter curtain opens, watch carefully as the second curtain closes 1 or 2 seconds later. If there's a problem with the second curtain you should be able to see it before the mirror drops down and the shutter resets. Don't blink, though, or you'll miss it!

To see the second curtain longer you can go to Live View and select Silent LV shooting mode 2 (see below). When you press the shutter button all of the way down hold it down and you should be able to see the second curtain completely covering the sensor. If there's a gap anywhere that lets you see part of the sensor at this point then the second curtain is faulty.

As far as having it repaired goes, it doesn't really matter. A shutter replacement will include both a new first and second curtain in the shutter assembly.

If it is only one of the curtains that is the problem there may be a workaround that will allow you to take images, albeit with greatly reduced functionality. You'll have to shoot using Live View from one of the Creative Zone exposure modes (P, TV, Av, M, or Bulb) and you'll give up the possibility of using any flash, even a non-Canon manual flash that doesn't communicate at all with the camera.

From within the Live View menu (the last red menu tab), select Silent LV shooting, press the Set button, select Mode 1 to avoid using the second shutter curtain or Mode 2 to avoid using the first shutter curtain, then press Set again. It's covered on page 231 of the EOS 70D Instruction Manual.

Notice the note at the bottom of that page: If you have a flash connected to the hot shoe that can be detected by the camera (generally any E-TTL capable flash or flash trigger that is turned on), the Silent LV shooting setting will be disabled. If you have a manual only flash connected to the hot shoe the hot shoe is disabled in Silent LV shooting and will not fire the flash.

If the 70D operates like the newer 7D Mark II, the second curtain closes to end the exposure in both mode 1 and mode 2 of Silent LV shooting and the above workaround will only work if the first curtain is the faulty one.

For more about how the Silent LV shooting modes work and the difference it makes to the shutter curtain sequence, please see this answer to: What's the difference between Canon's silent and non-silent shooting? Although that question is regarding the EOS 5D Mark III, the Live View silent shooting modes function similarly with the 70D (with the possible exception to which curtain is used in Silent LV shooting mode 1 as mentioned above).

I think the 70D has an “electronic second curtain” feature, off by default, hidden away in a menu somewhere. Find that and use it as a work-around!
– JDługoszFeb 2 '17 at 8:13

I'm sure this will indicate the problem. It wouldn't surprise me if the problem was even visible by using mirror lockup to inspect the closed shutter. Looking at the mechanical arrangement in your photo it's hard to see how one blade (and not even an outer blade) could stick -- it's more likely to be badly damaged. Having the shutter replaced by Canon looks viable.
– Chris HFeb 2 '17 at 10:49

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@JDługosz Some Canon DSLRs with a Silent LV shutter option use electronic first curtain. The first curtain, which is already open when in Live View, then becomes the effective second curtain by resetting to the default closed position without the second curtain ever moving. This is only the case in LV. No flash may be used in either silent LV mode. The silent shutter modes available when using the viewfinder all use the traditional two curtain sequence, the mirror is just moved at a slower speed.
– Michael CFeb 2 '17 at 11:23

After checking several of my EOS bodies, I discovered that the 2nd curtain is sometimes used to end the Silent LV shooting sequence. With the 5DII, 7D, and 5DIII the 1st curtain is used in Silent LV shooting mode 1 and the 2nd curtain is used with mode 2. With the 7DII both Silent LV shooting modes use the 2nd curtain to end the exposure.
– Michael CFeb 2 '17 at 15:58

I called Canon's tech support, and the guy there asked me if I was using a third-party battery. I am, and he asked what the voltage output was. I told him 7.4V, and he told me that was the problem. He claims that the 70D is supposed to have a 7.2V battery, and that this battery messed up my circuit board. I'm not sure I believe that it was because of this battery though. Anyway, I purchased a 5-year protection plan when I got my camera, so I was able to send it in for repair. I'll post when I hear back what the problem actually was.
– ironoxideyFeb 8 '17 at 7:37

from the shape of it, it might be an issue with the mirror not turning fully (perhaps stuck in such an angle that keeps reflecting something back onto the sensor)? This would explain also why it's fine with video. Maybe some grain of sand blocking its free movement?

If your camera supports it, you could test using electronic (silent) shutter for photos and see if it's there, this would help

He already checked for viewfinder light leaks. The silent shutter is a good idea: I do recall that the 70D has "electronic second curtain" feature, off by default.
– JDługoszFeb 2 '17 at 8:11

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No electronic second curtain in the 70D. Electronic first curtain only in Live View silent shooting mode, which also eliminates the use of flash (even a dumb manual flash that doesn't communicate with the camera at all).
– Michael CFeb 2 '17 at 11:25

Thank you @JDlugosz and Richard, I stand corrected and will rectify the post!
– FotisKFeb 2 '17 at 17:21